[Clang] Fix crash in coverage of if consteval.

Clang crashes when encountering an `if consteval` statement.
This is the minimum fix not to crash.
The fix is consistent with the current behavior of if constexpr,
which does generate coverage data for the discarded branches.
This is of course not correct and a better solution is
needed for both if constexpr and if consteval.
See https://github.com/llvm/llvm-project/issues/54419.

Fixes #57377

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D132723